# Definition for a binary tree node.
class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
class Solution:
    def invertTree(self, root: TreeNode) -> TreeNode:
        '''翻转二叉树'''
        if root is None:
            return root
        root.left,root.right=root.right,root.left
        self.invertTree(root.left)
        self.invertTree(root.right)
        return root